It has beem claimed that in old men levels of free serum testosterone decline sooner than do levels of total testosterone, but the finding has not been related to sexual activity in old men. In the coming year we propose to complete our analysis of free and bound testosterone in 20 old and 10 young adult male rhesue macaques and to correlate hormone levels with the level of sexual performance. We will examine the effects of pairing old males with preferred female partners on total sexual activity and the effects of test duration on the frequency of occurrence of selected sexual behaviors; we will determine whether frequency of sexual behavior is associated with serum levels of testosterone, dihydrotestosterone, and other steroid hormones. As part of our overall objective, we will continue to trace the decline in sexual performance associated with increasing old age and will correlate the decline with serum levels of hormone.